Multicast IP (Internet Protocol) technology simultaneously delivers a single stream of packets to an arbitrary number of receivers. Multicast IP is used in situations where a large number of terminals are to connect to a stream of data, such as in an IP based TV system where a large number of subscribers may be watching a live transmission at the same time. By using multicast IP, only a single copy of a packet is created on each subnetwork (or subnet) where there is a subscriber who wishes to receive that packet stream. When there is no subscriber in that subnet, the router will not forward the multicast packets into the subnet. Thus, multicast IP technology conserves bandwidth by replicating the packet stream only as required.
The alternative to using multicast IP in the network is that a unique copy of each packet is sent to each terminal. This requires that the IP header on each packet be created uniquely for each terminal, and the number of users watching that particular stream times the bandwidth for is required. This unicast mode of operation likely will cause problems during popular events, such as the Super Bowl, since the network would need to be over-provisioned to support that number of subscribers, even though that number of streams will rarely actually be in use.
In a fixed access network environment, such as found in a closed/managed cable television system, multicasting is effective for reducing the amount of traffic carried on the network. Because the service delivery network is private, issues of collisions in the address space reserved for multicast streams are not likely and the IPTV operator of the network has complete control over the multicast operation within the network. When a terminal, such as a laptop wishes to access these private multicast packet streams/media content from a public access network, however, this situation no longer holds. In other words, the laptop desires to take advantage of efficient multicast transmission of IPTV services. However, the public access network may not support multicasting. Furthermore, even if multicasting is supported, the multicast transmission may be susceptible to multicast address collision.